One solution for "Javascript is required to view this map." in Drupal 6 GMap module

I've been using GMap module for Drupal 6 for a while. Once I changed the theme from Garland to my own. The map page said only "Javascript is required to view this map".

I googled and found that I should include a javascript tag via drupal_add_js() function, but the scripts are already there when I did view source. Comparing with the working source, the missing piece is

<script src="" type="text/javascript"></script>

which is not included in a Drupal variable $script.

After comparing the page.tpl.php of the two themes line-by-line, the problem is that there was no $head in my new theme. I just inserted

<?php print $head ?>

before the line

<?php print $styles ?>

and the map appears.